In nuclear fuel bundles for boiling water reactors, assembly of the fuel bundle usually occurs around the water rods associated with such fuel bundles. To set the stage for this invention, the construction and operation of such a fuel bundle will first be summarily set forth. Second, the function of the large central water rod will briefly be delineated. Finally, the construction of the "tree" consisting of the spacers and the large central water rod will be discussed.
Nuclear fuel bundles for boiling water nuclear reactors typically include a lower tie plate for supporting a matrix of upstanding fuel rods of the bundle and for admitting water. An upper tie plate is secured to the other end of the fuel rods and enables the water and generated steam to exit the fuel bundle. Since the fuel rods are long and slender, they must be braced intermittently with respect to the tie plates. This being the case, normally seven so-called spacers are placed between the upper and lower tie plates. These spacers individually surround each fuel rod at the elevation of each spacer to prevent the fuel rods from abrading contact during the generation of steam and to maintain the design clearances between the fuel rods during bundle operation.
Typically, fuel bundles include large, central water rods. These water rods exceed by many times the diameter of the fuel rods and have for their main purpose the introduction of moderating water into the upper two phase region of the fuel bundle. Specifically, and during the generation of steam, the upper portion of a fuel bundle in a boiling water reactor has the steam fraction present in the fuel bundle predominate over the water fraction. This excess of steam renders the nuclear performance of the upper portion of the fuel bundle in a boiling water reactor deficient; more moderating water is needed in the upper two phase region of the fuel bundle for efficient nuclear performance. Hence, it has become a common practice to insert large central water rods.
These large central water rods attach at the lower tie plate, and communicate pure water from the lower portion of the fuel bundle to the upper portion of the fuel bundle. Thus, the water rods supply needed moderator to the upper two phase region of the fuel bundle.
As large central water rods have found their way into almost all boiling water nuclear fuel assembly designs, they have also entered into the construction of such fuel bundles. In the assembly of such fuel bundles, the spacers are first attached to the large water rods and registered at their respective water rod surrounding apertures to form a "tree" configuration. Thereafter, the individual fuel rods are threaded through the spacers at each successive fuel rod surrounding aperture to and towards one of the tie plates. The threaded rods pass successively through each rod aperture in each successive spacer until all such fuel rods are held firmly in place. Once all the fuel rods are in place, the remaining tie plate is attached and the fuel assembly--consisting of the tie plates, large central water rod, spacers, and fuel rods is ready for placement into the fuel channel.
Large central water rods are usually circular in cross-section; in the case under consideration here, the large central water rods are rectilinear in section--preferably square in cross-section.